The present invention relates to a cathode ray tube having a shadow mask, which is used for a television receiver, a computer display, and the like.
FIG. 5 is a cross-sectional view showing one example of a conventional color cathode ray tube. A color cathode ray tube 1 shown in FIG. 5 includes a substantially rectangular-shaped face panel 2 having a phosphor screen 2a on its inner face, and a funnel 3 connected to the rear side of the face panel 2. An electron gun 4 is contained in a neck portion 3a of the funnel 3, and a deflection yoke 5 is provided on the outer periphery of the funnel 3 in order to deflect and scan electron beams.
Furthermore, a shadow mask 6 is provided, opposed to the phosphor screen 2a, and a color-selecting electrode 9 is formed by fixing the shadow mask 6 to a pair of mask frames 7 held by a support 8. 10 indicates a track of electron beams.
The shadow mask 6 has a flat plate provided with a number of apertures formed by etching, through which electron beams pass, and plays a role of selecting colors with respect to three electron beams emitted from the electron gun 4.
In a color cathode ray tube, due to the thermal expansion caused by the impact of the emitted electron beams, the electron beam through apertures are shifted. Consequently, a doming phenomenon occurs. That is, the electron beams passing through the electron beam through apertures fail to hit a predetermined phosphor correctly, thus causing unevenness in colors. Therefore, a tension force to absorb the thermal expansion due to the temperature increase of the shadow mask 6 is applied in advance, and then the shadow mask 6 is stretched and held to the mask frame 7.
When the shadow mask 6 is stretched and held as mentioned above, even if the temperature of the shadow mask 6 is raised, it is possible to reduce the amount of displacement between an aperture of the shadow mask 6 and phosphor stripes of the phosphor screen 2a. 
However, the conventional color cathode ray tube suffers from the following problem. FIG. 6 is a perspective view of the color-selecting electrode 9 shown in FIG. 5. The shadow mask 6 is stretched and held to the mask frame 7 in a state in which the tension force is applied in the direction indicated by arrow Y. The shadow mask 6 has an effective area 11, in which a number of apertures 13 serving as electron beam through apertures are formed, and a dead space 12 on both sides in a horizontal direction thereof. In the effective area 11, the apertures 13 are neighboring in the vertical direction (vertical direction of the screen) via a bridge 14 and arranged in lines. Furthermore, the dead space has a width and a curvature at the end portions to some degree so that an appropriate tension distribution is provided over the shadow mask.
With regard to the shadow mask 6 illustrated in FIG. 6, due to the thermal expansion of the shadow mask 6 caused by the impact of the emitted electron beams, for example, in an area 15, which is a portion between the horizontally neighboring aperture lines, stress is applied in the direction indicated by arrows a. When such stress is applied, wrinkles are created in the area 15 and the aperture 13 is shifted in the horizontal direction. When such a so-called local doming phenomenon occurs, electron beams do not hit the shadow mask correctly, thus causing displacement of colors, unevenness in colors, and deterioration of luminance.
In addition, since the apertures are not formed in the dead space 12, the degree of the thermal expansion in the dead space is larger than that in the effective area 11 where the apertures 13 are formed, and thus, the aperture line adjacent to the dead space 12 is shifted by the difference of this thermal expansion. Therefore, the aperture lines adjacent to the area 12 have a larger degree of movement due to the local doming phenomenon.
Such a local doming phenomenon could not be prevented sufficiently even by stretching and holding the shadow mask as described above.
It is an object of the present invention to solve the conventional problem described above by forming a slit in the dead space of a cathode ray tube for preventing incorrect hitting of color electron beams caused by the local doming phenomenon from occurring and thus preventing displacement of colors, unevenness in colors, and deterioration of luminance from occurring.
To achieve the above object, a cathode ray tube of the present invention comprises a shadow mask having an effective area and a dead space formed on both outer sides of said effective area in a horizontal direction, a plurality of aperture lines having a plurality of apertures for passing electron beams being arranged via a bridge in said effective area, and the shadow mask being stretched and held in a vertical direction, wherein a slit extending along said aperture line is formed in said dead space. According to the cathode ray tube described above, the thermal expansion can be absorbed in the slit portion, so that the stress applied to the aperture line of the effective area adjacent to the dead space can be suppressed.
In the aforementioned cathode ray tube, it is preferable that a horizontal width of said slit is from 45% to 100% of a horizontal width of said aperture adjacent to said dead space. According to the cathode ray tube described above, a drastic difference is eliminated between the mechanical strength in the dead space and the mechanical strength in the effective area, so that it is possible to prevent bridges from tearing in the vicinity of the dead space or wrinkles from arising in the shadow mask.
Furthermore, it is preferable that a vertical length of said slit is equal to or longer than a vertical length of said aperture adjacent to said dead space. According to the cathode ray tube described above, the thermal expansion can be absorbed more surely by the slit.
Furthermore, it is preferable that said slit includes a slit having inclined faces opposed to each other via an opening, the inclined faces being formed at an inclined angle such that light beams of electron beams entering said dead space are blocked. According to the cathode ray tube described above, the light beams of the electron beams are blocked in a portion where the slit is formed, so that as far as the passing of the light beams is concerned, it is substantially the same as the shadow mask in which slits are not formed.
Furthermore, it is preferable that an electron shield is disposed for blocking said electron beams and thus preventing said electron beams from reaching said dead space. According to the cathode ray tube described above, since the electron beams do not hit the dead space directly, it is possible to suppress a temperature increase of the shadow mask.